1. Field of the Invention
This invention relates generally to the disposal of radiologically contaminated metallic components, and more specifically, to a method and apparatus for reducing the volume of radiologically contaminated metal racks used to store nuclear fuel assemblies.
2. Prior Art
In order to maintain optimal power generating capacity, the fuel used in nuclear reactors is periodically replaced with new fuel. The fuel that is removed from the reactor is highly radioactive, irradiated, and is referred to as "spent fuel". The spent fuel is placed into metal tubular cells usually rectangular tubes adjoined together to form a storage device that is referred to as a "spent fuel rack". Spent fuel racks are pre-positioned at the bottom of a deep pool of water located adjacent to the reactor, known as the "spent fuel pool". Spent fuel constitutes high level radioactive waste as that term is used in both a technical and a regulatory sense.
When most nuclear reactors were designed and built, it was assumed that spent nuclear fuel would be reprocessed. Based upon this assumption, nuclear reactor installations have historically been designed and constructed to accommodate temporary storage of spent fuel in the spent fuel pool. It was intended that the spent fuel temporarily stored in the spent fuel pool would periodically be removed for reprocessing thereby making space available to accommodate the ongoing discharge of spent fuel from the reactor. However, as the result of technical and political pressures within the United States, high level radioactive waste disposal and/or reprocessing capacity has never become commercially available. As a result, spent fuel pools that were originally designed only for the temporary storage of spent fuel are now being used for long term, indefinite storage.
The spent fuel stored within these racks occurs after the racks have been pre-positioned at the bottom of a deep pool of water that is located immediately adjacent to the nuclear reactor. The racks become highly contaminated because they are in physical contact with irradiated spent fuel and because contaminated pool water permeates the surface area of the metal of the racks. In order to maximize the storage capacity of pools, operators of nuclear reactors have replaced the racks with newer ones having updated space efficient designs. Following replacement, the old racks must be disposed as low level radioactive waste. Both federal law and economic prudence dictate that the volume of the racks be reduced prior to disposal, storage, or beneficial reuse.
When old spent fuel racks are removed from the spent fuel pool, disposal or disposition of contaminated spent fuel racks is both technically difficult, and very costly. Because the racks are in physical contact with spent fuel and contaminated pool water, surface radioactive contamination levels are typically high when compared to other forms of metallic low level radioactive waste. Depending on the number of racks being replaced, there can be tens or hundreds of thousands of pounds of material requiring disposition.
It is known prior art to segment the racks using oxyacetylene or plasma arc cutting techniques. Torch cutting of contaminated spent fuel racks is extremely problematical for several reasons. Torch cutting yields dense concentrations of smoke that is heavily ladened with radioactivity and other potentially toxic gases, and particulate materials. Filtering the smoke, particulate, and vapors to levels that meet regulatorily mandated levels for worker safety is extremely difficult and costly. Smoke and particulate quickly clogs filtration systems thereby jeopardizing worker safety and the environment. Even with effective ventilation, workers are exposed to high concentrations of airborne radioactivity and other potentially toxic substances. Additionally, while using torches, workers must be in extremely close proximity to the racks, including the lower portions of the racks were radiation levels are the highest. As a result of working in such close proximity to the racks, radiation exposure to workers is high.
It is an object of the present invention to dismantle and remove tube sections safely and efficiently from a rack assembly used to store waste nuclear fuel assemblies incident to reducing the volume of contaminated material that must eventually be stored or processed, while simultaneously minimizing the radiological exposure of the operating personnel.
It is a further object of the present invention to provide a method and apparatus to dismantle a storage rack used to store nuclear fuel assemblies by dismembering the storage rack of individual storage cells and if desired further subdivide and crush the dismembered cell section to effect volume reduction through a shape change from tubular configuration to a generally plate-like configuration.
It is another object of the present invention to provide a method and apparatus for the volume reduction of irradiated nuclear fuel storage rack that is fabricated from tube or pipe structures, generally weldments by nature and typically consist of metal such as stainless steel or aluminum materials. These structures usually have limited access from their exteriors due to the cross-sectional dimensions of the storage rack. The reduction in volume of the nuclear fuel storage rack is to occur by the systematic mechanical dismantling of the structure from inside of the tubular storage cells.
It is a still further object of the present invention to mechanically cut spent fuel racks fabricated from tubes which can be round or square forming fuel storage cells that are attached to a base plate and welded together at the lead-in ends of the cells. This mechanical dismantling of the spent fuel rack is to occur by inserting an abrading or cutting apparatus into the hollow of a selected spent fuel cell to sever the cell wall generally at as each of its opposite ends from a weldment type fabricated unit including a base and a lead-in end of the storage rack.